Adding a place holder node when a null value is found in vrelationship function in neo4j

I have the following query that is working as expected in neo4j. The query matches and collects distinct FEATURE, THREAT, GAP, FACT, and SCENARIO nodes with their relationships based on a specific feature key, then uses APOC procedures to create virtual relationships between these nodes for visualization, ensuring each node pair has its correct relationship type.

MATCH (fe:FEATURE)-->+(t:THREAT)-->+(g:GAP) 
MATCH (g)<--+(fa:FACT) 
MATCH (s:SCENARIO)<--+(g)
where fe.key= "Test S3 Storage#347585d0-e092-4808-a492-7b7a38e04dcd"

// Collect existing relationships and nodes
WITH collect(DISTINCT [fe, t]) + collect(DISTINCT [t, g]) AS hases,
     collect(DISTINCT [fa, g]) AS requireses,
     collect(DISTINCT [s, g]) AS scenarioGaps

// Create virtual relationships
WITH COLLECT { 
        WITH hases 
        UNWIND hases AS pair 
        RETURN [pair[0], apoc.create.vRelationship(pair[0], 'HAS', {}, pair[1]), pair[1]] AS tuple 
     } 
     + 
     COLLECT { 
        WITH requireses 
        UNWIND requireses AS pair 
        RETURN [pair[0], apoc.create.vRelationship(pair[0], 'REQUIRE', {}, pair[1]), pair[1]] AS tuple 
     } 
     + 
     COLLECT { 
        WITH scenarioGaps 
        UNWIND scenarioGaps AS pair 
        RETURN [pair[0], apoc.create.vRelationship(pair[1], 'HAS_SCENARIO', {}, pair[0]), pair[1]] AS tuple 
     }
     AS tuples

UNWIND tuples AS tuple

RETURN tuple[0], tuple[1], tuple[2]

Here is the issue I am facing:

There is a GAP node in my graph that I am querying which does not have a scenario node.
I want to include that GAP node without a scenario in my result of this graph.
If it is not directly possible, it is okay to add a place holder node there which will not be created in the real database.

I am unable to understand which approach should I use.

Solution

Try using OPTIONAL MATCH instead of MATCH for the (s:SCENARIO)<--+(g) pattern:

OPTIONAL MATCH (s:SCENARIO)<--+(g)

That allows the query to proceed when g has no path to a SCENARIO node. In that situation, the OPTIONAL MATCH would produce a NULL s value.

If you do that, then you would have to also change your last COLLECT subquery to ignore pairs that have a NULL first value.

For example:

MATCH (fe:FEATURE)-->+(t:THREAT)-->+(g:GAP) 
WHERE fe.key= "Test S3 Storage#347585d0-e092-4808-a492-7b7a38e04dcd"

MATCH (g)<--+(fa:FACT) 
OPTIONAL MATCH (s:SCENARIO)<--+(g)

// Collect existing relationships and nodes
WITH collect(DISTINCT [fe, t]) + collect(DISTINCT [t, g]) AS hases,
     collect(DISTINCT [fa, g]) AS requireses,
     collect(DISTINCT [s, g]) AS scenarioGaps

// Create virtual relationships
WITH COLLECT { 
        WITH hases 
        UNWIND hases AS pair 
        RETURN [pair[0], apoc.create.vRelationship(pair[0], 'HAS', {}, pair[1]), pair[1]] AS tuple 
     } 
     + 
     COLLECT { 
        WITH requireses 
        UNWIND requireses AS pair 
        RETURN [pair[0], apoc.create.vRelationship(pair[0], 'REQUIRE', {}, pair[1]), pair[1]] AS tuple 
     } 
     + 
     COLLECT { 
        WITH scenarioGaps 
        UNWIND scenarioGaps AS pair
        WITH pair
        WHERE pair[0] IS NOT NULL
        RETURN [pair[0], apoc.create.vRelationship(pair[1], 'HAS_SCENARIO', {}, pair[0]), pair[1]] AS tuple 
     }
     AS tuples

UNWIND tuples AS tuple

RETURN tuple[0], tuple[1], tuple[2]

Note that I also moved the top WHERE clause immediately under the first MATCH, to filter for the desired fe.key as early as possible.